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Why do you want to change the refresh token? After logged in, if the access token expired, you can call $rcsdk->platform()->refresh() to refresh the access token. If the refresh token also expired, you just need to call $rcsdk->platform()->login($option) again.

You can always check if the access token and the refresh token is still valid or not to decide what to do by calling $rcsdk->auth()->accessTokenValid() or $rcsdk->auth()->accessTokenValid(), respectively.

Can you verify that your app has the "Refresh Access Token" flow grant which is required to refresh your token? I will look like the following in the "Settings" > "OAuth Settings" > "Authorization Flows" section of your app configuration as shown below:

Yes, I received this error, there was a misprint in the link when copied), thank you very much, I'm just shocked, I sat for the whole evening and could not understand the problem, why not make a password check and report this error

If I was you, I would choose Ruby instead of PHP for Proof of concept. You have to rewrite the code in zoho deluge anyway. So you just choose the language which could help you to get things done very quickly.

I am not telling you that Ruby is quicker to write than PHP. Because we have an official Ruby SDK which is 10 times simpler than the PHP SDK. (Disclaimer: I am the author of the Ruby SDK while I am not the author of the PHP SDK).